Help with Java Thread for animation

[Phil Program]

My assignment is to change my program that animates race cars using a timer based on speed entered into text fields, to a program that uses threads to control the animation.

This is the problem I'm having: I do not fully understand how threads are implemented into code and where, but my program only works for two cars - sometimes one. Also, in the console I get NullException errors.

I would like to know how to properly code Java using threads to control the animation.

[GregBrannon]

Welcome to the forum! Please read this topic to learn how to post code in code or highlight tags and other useful info for new members.

Users prefer to see relevant code posted properly as described at the above link.

Swing is not thread safe. Sometimes mixing threads and Swing works, sometimes it doesn't. One way to ensure the mixing won't work is to use a while ( true ) loop with a Thread.sleep() to control the application's action.

The other preferred, recommended, correct approach is to use a javax.swing.Timer to control animation in a Swing app. There are at least 2 ways I can think of to accomplish what you're trying to do; one is with multiple Timers (one for each car), the other with just one. Either way, successfully moving one of the cars with one Timer is a start, so try that and come back.

[Phil Program]

Well it wasn't a lot of code, but okay thank you.

[Phil Program]

Everything runs fine, but I'm fairly new to Java and wanted to get some opinions.

Overview: This program is an animation of four racing cars - the speed is set by the user(no less than 1 or greater than 5). Initially using a timer with a private class to fire a fixed rate, our assignment was to use threads.

Did I implement thread usage correctly to control the car animation?

What can I do to improve my code to increase performance, effectiveness, or anything in general?

Looking forward to your responses!

import javax.swing.*;
import java.awt.*;
import java.awt.FlowLayout;
import javax.swing.border.*;
import java.awt.event.*;
 
public class RaceCarsThread extends JApplet {
 
	//create a new car object
	private final DrawCar car1 = new DrawCar();
	private DrawCar car2 = new DrawCar();
	private DrawCar car3 = new DrawCar();
	private DrawCar car4 = new DrawCar();
 
	//create textfields for each car for the users to specify their speed
	private final JTextField jtfSpeed1 = new JTextField(5);
	private final JTextField jtfSpeed2 = new JTextField(5);
	private final JTextField jtfSpeed3 = new JTextField(5);
	private final JTextField jtfSpeed4 = new JTextField(5);
 
 
	//creates a button labeled "start" to initiate the race
	private final JButton jbtStart = new JButton("Start");
 
	//stores the input for the speed of each car
	private int carSpeed1;
	private int carSpeed2;
	private int carSpeed3;
	private int carSpeed4;
 
        private Thread animator;
 
 
	public RaceCarsThread() {
 
 
		// Creates the top panel of the applet which stores holds the text fields for each car
		// along with the start button to begin the race.
		JPanel p1 = new JPanel();
		p1.setLayout(new FlowLayout(FlowLayout.CENTER));
		p1.add(new JLabel("Car 1:"));
		p1.add(jtfSpeed1);
		p1.add(new JLabel("Car 2:"));
		p1.add(jtfSpeed2);
		p1.add(new JLabel("Car 3:"));
		p1.add(jtfSpeed3);
		p1.add(new JLabel("Car 4:"));
		p1.add(jtfSpeed4);
 
 
 
		//sets the layout of the overall applet window
		setLayout(new GridLayout(6, 1));
		add(p1); //adds the first panel to the top of the window
                add(jbtStart);
		add(car1); // adds the first car to the window
		add(car2); // adds the second car to the window
		add(car3); // adds the third car to the window
		add(car4); // adds the fourth car to the window
 
 
 
 
	}
 
	public static void main(String[] args) {
		JFrame frame = new JFrame("Racing Cars");
		RaceCarsThread applet = new RaceCarsThread();
		frame.add(applet);
 
		frame.setSize(800, 350);
		frame.setLocationRelativeTo(null);
		frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
 
                frame.setVisible(true);
                applet.startCars();
 
                }
                 public void startCars() {
 
		// listens for the start button to be clicked
		jbtStart.addActionListener(new ActionListener() {
 
 
                        @Override
			public void actionPerformed(ActionEvent event) {
 
 
				carSpeed1 = Integer.parseInt(jtfSpeed1.getText());
                                carSpeed2 = Integer.parseInt(jtfSpeed2.getText());
                                carSpeed3 = Integer.parseInt(jtfSpeed3.getText());
                                carSpeed4 = Integer.parseInt(jtfSpeed4.getText());
 
 
 
				//to ensure that the speed of the car doesn't exceed the specified range of 1 to 5 
				if (carSpeed1 > 5) {
					carSpeed1 = 5; //if the user picks a number higher than 5 set the speed to 5
				} else if (carSpeed1 < 1) {
					carSpeed1 = 1; //if the user picks a number lower than 1 set the speed to 1
				}
 
			}
                    });
                 }  
 
	class DrawCar extends JPanel implements Runnable {
                private Thread timer = new Thread(this);
		private int xOrigin = 0; // holds the x coordinate of the car
 
		public DrawCar() 
                {
                    timer.start();
 
 
			setBorder(new LineBorder(Color.black, 2)); //draw a black line around the border of each cell that holds the car
 
                }
                @Override
                public void run(){
                                try {
        while (true) {
          repaint();
          Thread.sleep(10);
        }
      }
      catch (InterruptedException ex) {
      }
    }
 
 
                @Override
		protected void paintComponent(Graphics g) {
			super.paintComponent(g);
 
			// Create a Polygon object
			Polygon rectanglePolygon = new Polygon();
			Polygon trapezoidPolygon = new Polygon();
 
			// Add points to the tapazoidPolygon
			trapezoidPolygon.addPoint((xOrigin + 40), 18);
			trapezoidPolygon.addPoint((xOrigin + 30), 18);
			trapezoidPolygon.addPoint((xOrigin + 20), 30);
			trapezoidPolygon.addPoint((xOrigin + 50), 30);
			// Set the color for the trapezoid to red and fill in the
			// polygon
			g.setColor(Color.RED);
			g.fillPolygon(trapezoidPolygon);
 
			// Add points to the rectanglePolygon
			rectanglePolygon.addPoint((xOrigin + 10), 30);
			rectanglePolygon.addPoint((xOrigin + 10), 41);
			rectanglePolygon.addPoint((xOrigin + 60), 41);
			rectanglePolygon.addPoint((xOrigin + 60), 30);
			// Set the color for the rectangle to green and fill in the
			// polygon
			g.setColor(Color.GREEN);
			g.fillPolygon(rectanglePolygon);
 
			// Draw the circles for the wheels of the car
			g.setColor(Color.BLACK);
			g.fillOval((xOrigin + 20), 41, 10, 10);
			g.fillOval((xOrigin + 40), 41, 10, 10);
 
                        car1.xOrigin += carSpeed1; //increase the position of the car by what number the user specified as the speed
			car2.xOrigin += carSpeed2;
                        car3.xOrigin += carSpeed3;
                        car4.xOrigin += carSpeed4;
 
 
                        if(car1.xOrigin > 800 && car2.xOrigin > 800 
                                && car3.xOrigin > 800 && car4.xOrigin > 800)
                        {
 
                           carSpeed1 = 0;
                           carSpeed2 = 0;
                           carSpeed3 = 0;
                           carSpeed4 = 0;
 
                            car1.xOrigin = 0;
 
                            car2.xOrigin = 0;
 
                            car3.xOrigin = 0;
 
                            car4.xOrigin = 0;
 
 
 
 
                        }
 
                    }
		}
 
 
 
}

[Norm]

Threads merged.

Is this an applet or a desktop program? It extends JApplet AND has a main() method. They don't go together.

[Phil Program]

Threads merged.

Is this an applet or a desktop program? It extends JApplet AND has a main() method. They don't go together.

Well...this is embarrassing, but it was an assignment that was supposed to be an applet. But I'm new to Java - I went from Objecct Oriented C++ to Advanced Java, so I have no foundation.

It doesn't make a difference though in terms of the threads though does it? How can I make it just a desktop program, and how can I make it just an applet though

[Norm]

The GUI can be handled in a JPanel and that JPanel can be displayed in a applet or in a desktop JFrame.

See the tutorial for how to code an applet: Lesson: Java Applets (The Java™ Tutorials > Deployment)

[Phil Program]

Great - the main point though - is the program running okay with the threads? My assignment is for the threads to control animation.

[Norm]

It works for me.

[priya456]

Make a class to represent an oval. Make two instances. Give the two instances different angular velocities. Currently because you increment degrees by 1.0 every 25 ms you have an angular velocity fixed at 40 degrees per second. If each oval has its own degrees field and you increment the two by different amounts, the ovals will rotate at different rates.